Retroreflective article comprising a transparent base sheet and nacreous pigment coating, method for making such a base sheet, and method for making a forming master

ABSTRACT

A retroreflective article comprising a transparent base sheet having first and second broad faces, the second face being substantially planar and the first face having an array of substantially hemi-spheroidal microlenses thereon, the microlenses being wider and shorter than portions of true spheres, the shape of the microlenses and thickness of the base sheet being such that collimated light substantially orthogonally incident to the array is focused approximately at the second face, wherein the second face has a coating of a nacreous pigment composition thereon. The article can include an optional layer of adhesive over the reflective layer to permit the article to be adhered to a document as a tamper-indicating, authenticating, and protective overlay. Special images can be formed within the article to enhance the security provided thereby. Also, a method for making such base sheets, and a method for making a forming master.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application No. 08/099,184, filed Jul. 29, 1993, now U.S. Pat. No. 5,468,540, issued Nov. 21, 1995, which is a continuation-in-part of application Ser. No. 07/613,136, filed Nov. 15, 1990, now U.S. Pat. No. 5,254,390 issued Oct. 19, 1993.

FIELD OF THE INVENTION

The invention concerns retroreflective articles, e.g., used as transparent overlays to protect documents from tampering, comprising a novel base sheet and a nacreous pigment coating thereon. The invention also concerns a method for making such base sheets and a method for making a forming master.

BACKGROUND OF THE INVENTION

Documents often have adherent transparent overlays to provide protection against dirt, moisture, and general wear and tear. A typical protective transparent overlay has a plastic film bearing an aggressive adhesive layer by which it can be permanently adhered to the face of a document. A transparent overlay which does not obscure underlying information is disclosed in U.S. Pat. No. 3,801,183 (Sevelin et al.). The patent discloses legend-containing sheet materials which have retroreflective legend and retroreflective background areas that are substantially indistinguishable under ordinary diffuse light viewing conditions but are clearly distinguishable under retroreflective viewing conditions. Attempts to tamper with information over which the overlay has been applied result in readily detectable disruption of the overlay.

Subsequent to U.S. Pat. No. 3,801,183, a number of patents have issued disclosing other transparent overlays, each of which can be imaged with a pattern or legend that is readily noticeable only when viewed retroreflectively and can be adhesively bonded to a document without obscuring the face of the document. See, for example, U.S. Pat. No. 4,099,838 (Cook et al.) which discloses an overlay wherein the retroreflection from the background and image areas of the pattern is of contrasting colors. See also U.S. Pat. Nos. 4,688,894 (Hockert) and No. 4,691,993 (Porter et al.), which disclose transparent overlays that function similarly as those of U.S. Pat. No. 3,801,183 while having the added capability of permitting an authenticating message to be formed in the overlay after it has been adhesively bonded to a document.

U.S. Pat. No. 5,169,707 (Faykish et al.) discloses a transparent overlay having a primary legend which has a readily detectable appearance under ordinary diffuse lighting conditions to provide a first level of screening or verification that can be carried out simply and conveniently without special equipment. A second level of verification can also be provided by the primary legend under retroreflective viewing conditions.

The transparent protective overlays disclosed in the above-cited patents each comprise a monolayer of glass microspheres that makes the overlay difficult to counterfeit but somewhat expensive to make.

U.S. Pat. No. 4,869,946 (Clay) discloses a security card comprising a transparent upper layer having narrow parallel lenses on its outer surface and an image-containing substrate on its inner surface. The two layers form a lenticular system by which images on the substrate are selectively viewable depending upon the angle from which the card is viewed.

U.S. Pat. No. 3,503,315 (de Montebello) discloses a lenticular plate or sheet comprising adjacent lenslets with spherical convex surfaces on both sides. The sheet is used for the production and display of a picture in panoramic stereoscopic relief.

U.S. Pat. No. 2,951,419 (Lemelson) discloses retroreflective devices which appear, to an observer moving with a light source, to flash on and off, scintillate, or change optical composition. In one embodiment, the device comprises a transparent sheet and a retroreflective sheeting which meet at an interface. At the interface, there is provided two groups of adjacent strips having different color or light reflectivity. At its front surface, the transparent sheet is formed with a number of lens-like formations. An observer moving with a light source sees alternating patterns of color as discrete bands of light from the moving light source enter the device in such a manner that the bands shift from one group of strips to the other group of strips and vice versa.

U.S. Pat. No. 4,576,850 (Martens) concerns shaped plastic articles made by replicating a microstructure-bearing surface with an ultraviolet-curable organic oligomeric composition.

A number of other patents also concern shaped plastic articles made by replicating a microstructure-bearing surface. For example, see U.S. Pat. Nos. 3,689,346 (Rowland); 4,414,316 (Conley); 4,420,502 (Conley); and 4,374,077 (Kerfeld).

SUMMARY OF THE INVENTION

The invention provides retroreflective articles, including for example, transparent protective overlays for documents. Such overlays can be imaged with information to enhance the authenticity of a document, e.g., a logo, and also provide high tamper-resistance. Some of the information can be in the form of so-called "flip-flop" images that are viewable only across a narrow range of angles and change color across that range of angles. Such images can be viewable in ambient light, thus affording a readily apparent verification of the authenticity of the document. A "flip-flop" image can afford a second level of authenticity, by becoming bright when viewed retroreflectively, thus making it especially difficult either to tamper with the document or to counterfeit the novel overlay.

In brief summary, the retroreflective articles provided herein comprise a base sheet having first and second broad faces wherein the second face is substantially planar and the first face has an array of substantially hemi-spheroidal microlenses thereon, the microlenses being wider and shorter than portions of true spheres. The shape of the microlenses and thickness of the base sheet are such that collimated light substantially orthogonally incident to the first face, i.e., the array, is focused approximately at the second face of the base sheet. The second face of the base sheet has a coating of a nacreous pigment thereon.

The invention also provides a method for making such basesheets. Briefly summarizing, this method comprises:

a) preparing a curable composition;

b) depositing the composition onto a forming master surface having an array of substantially hemi-spheroidal concavities, the concavities being wider and shorter than portions of true hemi-spherical concavities;

c) spreading a bead of the composition between a transparent substrate film and the forming master, the substrate film being substantially planar, at least one of the substrate film and the forming master being flexible;

d) curing the deposited composition to yield a composite comprising an array of substantially hemi-spheroidal microlenses bonded to the substrate film; and

e) removing the composite from the forming master to yield the base sheet.

In yet another of its aspects, the invention relates to a method for making a forming master which is adapted to be used in the above-described method. This method for making a forming master comprises:

a) providing an initial master comprising a retroreflective sheeting, the sheeting comprising a monolayer of microspheres and a space coat bearing a metallic coating and covering the monolayer of microspheres, the metallic coating comprising a surface having an array of substantially hemi-spheroidal protrusions thereon, the protrusions being wider and shorter than portions of true hemi-spherical protrusions;

b) preparing a curable composition;

c) depositing the composition onto the metallic coating in an amount at least sufficient to cover the protrusions;

d) spreading a bead of the composition between a substrate film and the initial master, the substrate film being substantially planar, at least one of the initial master and the substrate film being flexible;

e) curing the composition to yield a composite comprising the substrate film and the composition; and

f) removing the composite from the initial master to yield the forming master.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained with reference to the drawing. In the drawing:

FIG. 1 is a schematic cross section of a portion of a retroreflective sheeting of the invention;

FIG. 2 is a schematic cross section of a portion of one embodiment of a transparent protective overlay of the invention after it has been laminated to the face of a document;

FIG. 3 is a schematic cross section of a portion of another embodiment of a transparent protective overlay of the invention; and

FIG. 4 is a photomicrograph of the microlens-covered face of a base sheet of the invention.

FIGS. 1-3, which are idealized, are not to scale and are intended to be merely illustrative and non-limiting.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As shown in FIG. 1, in a typical retroreflective embodiment, article 10, e.g., reflective sheeting, comprises base sheet 11 having first and second broad faces, array 14 of substantially hemi-spheroidal microlenses on the first broad face, and substantially continuous specularly reflective layer 17 on the second face. Base sheet 11 is substantially transparent and has the proper refractive index and dimensions such that collimated light orthogonally incident to the array is focused approximately at the second face of base sheet 11, i.e., at specularly reflective layer 17.

The base sheets provided herein are referred to as "plano-convex". This refers to the fact that the second or rear face is substantially flat and planar whereas the first or front face has an array of convex microlenses thereon. The microlenses are typically substantially hemi-spheroidal. A spheroid is the figure generated by revolving an ellipse about one of its axes. As used herein, "hemi-spheroidal" refers to the fact that each microlens is only a portion of a spheroid. The axis of each microlens is substantially perpendicular to the plane of the base sheet at the base of the microlens. The perimeter of each microlens is widest at its base where it protrudes from the base sheet. The microlenses are generally very tiny, i.e., typically on the order of about 50 to about 150 micrometers wide at their bases. The surface of each microlens may conform to a portion of a true sphere, but is preferably proportionally wider and shorter than a portion of a true sphere to widen the width of the conical field in which incident light is focused to the second face of the base sheet. Typically, light which is incident to the array within a conical field oriented perpendicularly to the base sheet and having a full width of about 5 to about 10 degrees will be focused at the second face of the base sheet, and in retroreflective embodiments, will be retroreflected. Incident light within this orientation is referred to herein as substantially orthogonal.

Preferably, the microlenses are closely packed within the array to enhance the degree of focusing, and in retroreflective embodiments, increase the retroreflective brightness. In some embodiments, the microlenses may be so closely packed as to be substantially contiguous, i.e., the adjacent microlenses are in contact rather than being spaced apart. In some instances, adjacent microlenses may be partially overlapping as shown in FIG. 4. An advantage of the invention is that the microlenses need not be arranged in ordered fashion, i.e., no particular arrangement such as cubic or hexagonal arrangement is necessary. As shown in FIG. 4, the microlenses may be arranged in essentially random arrangement.

When the article is to be used as a document overlay, the reflective layer is preferably transparent. Illustrative examples of reflective layers include zinc sulfide, cryolite, lithium fluoride, silicon oxides, and magnesium fluoride. U.S. Pat. No. 3,700,305 (Bingham et al.) and the aforementioned U.S. Pat. No. 3,801,183 disclose the use of dielectric mirrors as partially light transmissive reflectors and are incorporated herein by reference. When the reflective layer is transparent, it can be continuous without obscuring any information on the face of a document to which the novel protective overlay is applied. However, the reflective layer can be opaque when it is discontinuous and applied so as to not mask information on the face of the document to which it is applied.

A pattern of imaging material, i.e., typically clear ink or varnish, may be applied to the second face of the base sheet before a dielectric mirror is applied thereover to provide a legend or legends within an overlay of the invention.

Instead of or in addition to a message or pattern of clear ink, the second face of the transparent polymeric layer (beneath the reflective layer) can be imprinted with an opaque ink. For example, when the overlay is to be used to protect a document that has been preprinted with information standard to all like documents, e.g., with boxes labeled to receive a bearer's name, address, and the like, the opaque ink printing can fill those boxes with information specific to the bearer. If someone were able to peel such an overlay from a document, it would carry with it at least some of the opaque ink coating. Then, to change the information, one would need to erase any part of that coating remaining on either the document or the overlay and add counterfeit information to fill the boxes. The task of doing so would be especially difficult when part of the ink coating is transparent and forms a so-called "flip-flop" image in a pattern that is easily disrupted upon tampering with the overlay.

To provide a "flip-flop" image, a message or pattern of clear ink is imprinted on the second face of the base sheet prior to application of a reflective layer thereover. The resultant change in nature of the surface of the base sheet tends to result in variations in thickness of the reflective layer when it is applied, thus producing an image that can repeat across the full face of the overlay and can be seen in ambient light with the naked eye across a narrow range of angles. The color of the image changes at differing viewing angles and is thus sometimes referred to as a "flip-flop" image. Any attempt to peel back the overlay to change underlying information is likely to disrupt the colored "flip-flop" image and thus alert anyone who looks at the document. Because the colored "flip-flop" image is bright when viewed retroreflectively, a retroreflective examination can provide secure proof that the document is authentic.

FIG. 2 illustrates another embodiment, wherein overlay 20 comprises base sheet 21 which consists of two strata, i.e., transparent spacing layer 22 and array 24 of microlenses. An advantage of such multi-strata embodiments is that the materials used for spacing layer 22 and array 24 may be independently selected to optimize the overall properties of the article. For instance, a high performance casting material which is used to make the array may be relatively expensive so that a cheaper material is used in the spacing layer to reduce the overall cost of the article. In another example, in the case of a document overlay, the array can be selected to be very hard and resistant to abrasion to impart greater durability whereas the spacing layer is selected to have a relatively low tensile strength to make its intact removal from a document more difficult.

Referring again to FIG. 2, on the rear face of spacing layer 22, i.e., the face opposite array 24, is discontinuous coating 26 of clear ink over which is continuous transparent reflective layer 27. Covering reflective layer 27 is adhesive layer 28 which may be used to secure overlay 20 to the face of document 29 over information 29A printed thereon.

Because retroreflective material 27 adhered better to the areas of clear ink 26 than it did to uninked areas of spacing layer 22 during application, e.g., by sputtering, it is thicker at the inked areas. The resultant variations in thickness of the transparent reflective layer 27 produce a colored "flip-flop" image that can be seen in ambient light through the polymeric layer, which image is bright when viewed retroreflectively.

Many different adhesives may be used on articles of the invention, including, for instance, hot melt adhesives, pressure-sensitive adhesives, and actinicly activated adhesives, e.g., heat-activated and ultraviolet-activated adhesives. In the case of document overlays, the adhesive should be substantially transparent.

In some embodiments, document overlays may be made with a layer of adhesive disposed between the second face of the base sheet and the reflective layer as well as on the opposite side of the reflective layer as the base sheet. FIG. 3 shows such an embodiment wherein overlay 30 comprises transparent base sheet 31 consisting of spacing layer 32, array of microlenses 34, and transparent adhesive stratum 33. In such embodiments, light incident to array 34 is focused at the back side of adhesive stratum 33, i.e., the side opposite spacing layer 32. Applied to the back of adhesive 33, i.e., the face opposite base sheet 31, is pattern of ink 36 over which has been applied transparent reflective layer 37 that has adhered to ink 36 but not to adhesive 33. Covering reflective layer 37 is another layer of adhesive 38 by which overlay 30 can be adhered to the face of a document (not shown). When adhesive layers 33 and 38 are of identical composition, an attempt to remove overlay 30 from a document is likely to cause splitting of both layers of adhesive, thus making it especially difficult for anyone to tamper with the document and then replace the overlay without leaving a clear indication of the tampering.

In some embodiments, retroreflective articles of the invention comprise coatings of nacreous pigment particles or flakes (also known in the art as pearlescent pigment or interference pigment particles or flakes). Such nacreous pigment coatings can be applied to a substrate, typically in a pattern, such that the particles or flakes are substantially uniformly oriented, whereby the pattern exhibits discernibly different light-reflecting characteristics when viewed from different angles under ordinary diffuse lighting conditions. Such differing characteristics can include changes in color or opacity with changes in viewing angle. For instance, at certain angles sometimes referred to herein as specularly reflective viewing angles, the cumulative reflection of the multiple layers of flakes is greatest, typically being great enough that the aggregation of flakes in the pattern tends to block the view of any underlying matter. At these angles, the nacreous pigment coating tends to reflect incident light rays. In contrast, when that same pattern is viewed at other angles sometimes referred to herein as transmission viewing angles, the reflective efficiency of the flakes may be lower so that a greater portion of incident light is transmitted through the pattern, even through many layers of flakes. As a result, at these angles, the pattern appears fainter, perhaps substantially undetectable by the unaided eye. At these angles, the nacreous pigment coating tends to transmit incident light rays through the pattern to any underlying matter.

The nacreous pigment coating can be applied in any random or ordered arrangement, but typically preferably is applied in an ordered arrangement. For example, a coating of a nacreous pigment can be applied in a desired pattern by applying in imagewise fashion a desired nacreous pigment composition. The nacreous pigment composition is typically applied in a pattern which is preferably between about 2 and about 10 micrometers thick, although patterns having thicknesses outside this range may also be used.

Suitable nacreous pigment compositions typically preferably comprise nacreous pigment particles or flakes, a suitable vehicle, i.e., binder resin, typically transparent, a dye, and solvent or dispersing agents. If desired, the dye can be selected such that its color is dark enough relative to the color of the nacreous pigment particles that the dye masks the color of the nacreous pigment particles under ordinary diffuse light viewing conditions but does not mask the color of the nacreous pigment particles under retroreflective viewing conditions.

Illustrative examples of useful nacreous pigments include bismuth oxychloride, lead subcarbonate, titanium dioxide-coated mica, and guanine, with bismuth oxychloride typically being preferred. As will be understood by those skilled in the art, a sufficient amount of suitable liquid component would typically be used to facilitate preparation and handling of the composition. Many suitable liquids are known, including several organic solvents. Water is commonly preferred for environmental and cost reasons. It will be understood that other formulations can be used also. Further details regarding nacreous pigments can be found, for example, in the aforementioned U.S. Pat. No. 5,169,707 which is incorporated by reference herein in its entirety.

Typically, if nacreous pigment compositions are included in retroreflective articles of the invention, the nacreous pigment compositions will replace the ink of coatings 26 (FIG. 2) or 36 (FIG. 3) or the material of reflective layers 17 (FIG. 1), 27 (FIG. 2), or 37 (FIG. 3). Also, if desired, in the embodiment of FIG. 2, a nacreous pigment composition can replace both the ink of coating 26 and the material of reflective layer 27. If nacreous pigment compositions are used in coating 26 and layer 27, the compositions typically preferably differ in color so that the coating 26 and layer 27 have distinctly contrasting colors when viewed under diffuse lighting conditions.

Similarly, in the embodiment of FIG. 3, a nacreous pigment composition can replace both the ink of coating 36 and the material of reflective layer 37. If nacreous pigment compositions are used in coating 36 and layer 37, the compositions typically preferably differ in color so that the coating 36 and layer 37 have distinctly contrasting colors when viewed under diffuse lighting conditions.

As an illustrative example of the use of nacreous pigment coatings, referring to FIG. 2, article 20 can comprise discontinuous nacreous pigment coating 26 on its second broad face. Nacreous pigment coating 26 is typically provided in the form of a pattern on the second broad face. Continuous layer 27 is disposed over nacreous pigment coating 26. Layer 27 can be specularly reflective and opaque. Alternatively, the reflective layer can be discontinuous, e.g., the reflective layer can be in substantial registration with the nacreous pigment coating as shown in FIG. 3.

Another possibility is that the nacreous pigment coating could be substantially continuous. Also, an imaging material could be disposed between the second face of the base sheet and the nacreous pigment coating. The imaging material could comprise a second nacreous pigment coating with the first and second nacreous pigment coatings typically having distinctly different colors under ordinary diffuse lighting conditions.

An advantage of base sheets of the present invention is that they may be used to make retroreflective articles without utilizing a monolayer of relatively expensive microspheres. Retroreflective articles of the invention provide selective angularity as mentioned above and can be made relatively inexpensively. In the case of document overlays, any information and patterns printed between the back of the base sheet and the face of the document are protected by the base sheet. Resins used in forming the base sheet, and particularly the array of microlenses, can be selected to have excellent resistance both to abrasion and to weathering.

A base sheet composite that can be converted to retroreflective sheeting of the invention can be made by using, as an initial master, retroreflective sheeting that includes a monolayer of enclosed glass microspheres and a space coat bearing a metallic coating, e.g., the specularly reflective layer of FIG. 4 of U.S. Pat. No. 4,505,967 (Bailey). Such a metallic coating has a surface with hemi-spheroidal protrusions, the protrusions being wider and shorter than portions of true hemi-spherical protrusions and typically being substantially contiguous and arranged in a substantially random arrangement. A forming master can be made by the following method:

a) preparing a transparent curable composition, such as an ultraviolet-curable oligomeric composition, e.g., as disclosed in the aforementioned U.S. Pat. No. 4,576,850;

b) depositing that oligomeric composition onto the metallic coating of the initial master in an amount at least sufficient to cover the protrusions;

c) spreading a bead of the composition between a substrate film and the initial master, the substrate film being substantially planar, at least one of the substrate film and the initial master being flexible, as taught in the aforementioned U.S. Pat. No. 4,374,077;

d) curing the deposited composition, e.g., by ultraviolet radiation, to yield a composite of the substrate film and cured oligomeric composition; and

e) stripping the composite from the initial master to yield a forming master with a surface having an array of substantially hemi-spheroidal concavities therein, the concavities being wider and shorter than portions of true hemi-spherical concavities.

The aforementioned U.S. Pat. No. 4,374,077 discloses replication by polymerization of a liquid mass deposited between a shaped surface and a substrate to which the mass is to adhere and is incorporated herein by reference.

The composite with its surface of hemi-spheroidal concavities may be used as a forming master from which a base sheet of the invention is made. This can be done by performing a similar process utilizing the thusly made forming master and a substrate film or base film of proper thickness to act as a spacing layer. Illustrative examples of useful base films include flexible films of polyester, polyvinyl chloride, and polymethyl methacrylate. Alternatively, the forming master may be specially tooled to form a desired array of microlenses. It will typically be somewhat expensive to make forming masters in that fashion, an advantage of the technique described above being that a suitable forming master can be obtained rather economically.

The curable composition is selected in part for its processing properties, i.e., viscosity, curing conditions, etc., and its post-cure properties, i.e., abrasion resistance, clarity, etc. An illustrative class of curable compositions are the ultraviolet-curable compositions disclosed in the aforementioned U.S. Pat. No. 4,576,850 which is incorporated herein by reference. That patent discloses one-part, solvent-free, radiation addition-polymerizable oligomeric compositions containing hard and soft segments. The compositions typically contain between about 0.1 and about 0.5 percent by weight of photoinitiator. The hard segments preferably are polyurethane, and the soft segments preferably are polyester.

The curable composition is preferably applied to the forming master in limited quantity, e.g., such that the composition completely fills the hemi-spheroidal concavities but does not extend above the surface of the forming master by a distance greater than about 20 percent of the average depth of the concavities. In this fashion, the substrate film which becomes at least part of the spacing layer may be used to substantially define the desired overall tensile and film-like integrity of the article. Also, the curable composition is typically more expensive than the substrate film material, and it is typically easier to control shrinking and warping during curing if the curable mass is of reduced thickness. Preferably, the microlens array is between about 0.01 and about 0.05 millimeter in thickness when a retroreflective sheeting containing a monolayer of microspheres having an average diameter of about 65 micrometers is used as an initial master as described above. In such instances, the spacing layer is typically between about 0.2 and about 0.3 millimeters thick.

The viscosity of the oligomeric composition as deposited in step b) preferably is within the range of about 200 to about 2,000 centipoise ("cps"). Above that range, air bubbles might tend to be entrapped, and the resin might not completely fill the cavities of the master molding surface. Below that range, the resin would tend to shrink upon curing to such an extent that the cured resin may not faithfully replicate the master molding surface. Within the preferred range, the oligomeric composition should completely fill the master cavities without any need to apply pressure. More preferably, the viscosity of the resin is between about 200 and about 1,000 cps and most preferably is between about 300 and about 400 cps.

In order to achieve the desired viscosity in compositions such as disclosed in the aforementioned U.S. Pat. No. 4,576,850, it may be necessary to include in the oligomeric composition an ethylenically unsaturated monomer such as an alkyl acrylate, preferably an alkyl acrylate of which the alkyl group contains a straight chain of from 4 to 12 carbon atoms. The relative amounts of oligomer and monomer should be controlled so that the overall equivalent weight of the composition is not so low that there is unacceptably high shrinkage upon curing.

Polyvinyl chloride is a preferred substrate for use in step c) as it is typically economical, durable, transparent, and dimensionally stable. Other illustrative examples of useful substrates include polycarbonate, biaxially oriented polypropylene and biaxially oriented poly(ethylene terephthalate) films, the surfaces of which have been treated to promote adhesion to the oligomeric composition, e.g., by corona treatment. Any of those substrates, typically used in thicknesses between about 0.05 and about 0.4 millimeter, can be flexed to propagate a bead of resin as taught in the aforementioned U.S. Pat. No. 4,374,077. Additional illustrative examples of useful substrates include cellulose acetate butyrate, cellulose acetate propionate, poly(ether sulfone), poly(methyl methacrylate), and polyurethane films, and glass.

The curable composition and substrate are preferably selected that upon curing a strong bond is formed therebetween.

The optimum thickness of the substrate layer or spacing layer is dependent in part upon the average size and shape of the microlenses. In typical retroreflective sheeting such as used as an initial master as described above, 80 percent of the microspheres have a diameter within 15 percent of the average. When the average diameter of the microspheres is about 65 micrometers, the microlenses of the resultant base sheet of the invention have an approximate average focal length of between about 0.2 and about 0.3 millimeter, thus allowing a spacing layer to be between about 0.2 and about 0.3 millimeter in thickness, or somewhat thinner when a transparent adhesive layer is provided between the base sheet and reflective layer. Because a thinner substrate would be more economical and equally useful, it may be preferred to start with retroreflective sheeting having glass microspheres of smaller average diameter.

Articles of the invention may be made in substantially rigid or flexible form. For instance, with selection of suitable materials, articles of the invention may be so flexible as to be capable of being wound into roll form about a mandrel 1 centimeter in diameter.

EXAMPLES

The invention will be further explained by the following illustrative examples which are intended to be nonlimiting. Unless otherwise indicated, all amounts are expressed in parts by weight.

RETROREFLECTIVE BRIGHTNESS

Retroreflective brightness was measured at a 0.2° divergence angle and a -4° entrance angle in candellas per lumen using a retroluminometer as described in United States defensive publication T987,003.

EXAMPLE 1

A one-part, solvent-free, radiation addition-polymerizable, crosslinkable, organic oligomeric composition having hard segments and soft segments and a viscosity of 1600 cps was prepared by blending together the following components:

    ______________________________________                                                               Parts                                                    ______________________________________                                         acrylate-capped polycaprolactone urethane                                                              54.3                                                   oligomer                                                                       N-vinyl pyrrolidone     16.3                                                    (ethoxy)-2-ethoxy!ethyl acrylate                                                                      11.3                                                   1,6-hexanediol diacrylate                                                                              5.7                                                    N-(isobutoxy methyl)acrylamide                                                                         11.1                                                   tertiary amine mixture (TINUVIN 292 from                                                               1.0                                                    Ciba-Geigy)                                                                    1-hydroxy cyclohexyl acetophenone                                                                      0.25                                                   ______________________________________                                    

Used as an initial master surface was the metallic reflective layer of an enclosed-lens retroreflective sheeting comprising a monolayer of glass microspheres having an average diameter of about 65 micrometers. Onto the metallic layer was deposited the liquid oligomeric composition in an amount barely sufficient to fill the cavities of the metallic layer. This was overlaid with clear polycarbonate film 0.125 millimeter in thickness through which the oligomeric composition was irradiated using 2 passes of a medium-pressure mercury lamp of 350 to 380 nanometers, 125 watts per linear centimeter, thus providing a total exposure of 110 millijoule/centimeter². The contoured surface of the resulting first composite consisted of substantially contiguous hemi-spheroidal concavities.

The same procedure was repeated using the resultant composite as a forming master, thus producing a second composite having a total thickness of about 0.2 millimeter and consisting of a polycarbonate substrate and an array of substantially hemi-spheroidal microlenses.

Over the flat surface of the second composite, aluminum was applied by vacuum deposition to a thickness of about 70 nanometers. The resulting retroreflective sheeting had a retroreflective brightness of 390 candellas per lumen.

EXAMPLE 2

A composite was made like the second composite of Example 1 except that the thickness of its polycarbonate substrate was 0.25 millimeter. The photomicrograph of FIG. 4 shows the resultant microlens array.

Onto the flat surface of the polycarbonate substrate was screen-printed a pattern of colorless ink (RAGE-800 Ink from Advance Process Supply of Chicago) that dried to a thickness of less than 0.01 millimeter. Over this, zinc sulfide was applied by vacuum deposition to a thickness of about a quarter wavelength at 520 nanometer to provide a transparent reflective layer. The ZnS tended to stick to only the clear ink and the areas immediately around the ink deposits, leaving the exposed areas of polycarbonate uncovered. A 0.05 millimeter thick layer of ELVAX 550, an ethylene/vinyl acetate copolymer hot melt adhesive from Du Pont was then laminated thereover. The resulting transparent protective overlay of the drawing was laminated by its adhesive layer at 82° C. to the face of a driver license bearing printed information.

When viewed through the overlay, that printed information was clearly readable in spite of a faintly colored "flip-flop" image where the clear ink had been applied, which image was in a pattern extending over the whole face of the driver license and was clearly visible in ambient light. When viewed retroreflectively, the pattern was bright.

The microlenses were rubbed with fine sandpaper to simulate the wear that could be expected over a period of several years in normal use, after which the printed information remained readable and the colored "flip-flop" pattern remained clearly visible in ambient light.

EXAMPLE 3

A driver license was made in the same manner as in Example 2 except as follows:

a) the substrate was polyvinyl chloride;

b) two layers of the adhesive were applied as illustrated in FIG. 3;

c) a pattern of the colorless ink was screen-printed onto the exposed face of the adhesive layer that was applied to the second face of the base sheet;

d) ZnS was applied over this pattern and adhered only to the ink and not to the adhesive.

When this driver license was viewed through the overlay, a "flip-flop" image like that of Example 2 was visible in ambient light and was bright when viewed retroreflectively.

EXAMPLE 4

A composite (about 15×28 centimeters) was made like the second composite of Example 1 except that the substrate was polyvinyl chloride 0.25 millimeter in thickness, and it was converted to retroreflective sheeting by applying ZnS by vacuum deposition to a thickness of about a quarter wavelength at 520 nanometers. Retroreflective brightness measurements were made at each of the four corners and at the midpoints along the long sides. Results, in candellas per lumen, were:

    ______________________________________                                         68               78    71                                                      56               71    69.                                                     ______________________________________                                    

In view of the fact that the thermoplastic polyvinyl chloride film inevitably had variations in thickness which resulted in a somewhat non-uniformly thick spacing layer and that the microlenses varied in size, these values are remarkably uniform.

EXAMPLE 5

A composite was prepared as in Example 2 except that the colorless ink was replaced by a colored transparent ink formulated as follows: 80 parts of colorless ink (Roto/Flexo Extender FA-1626 from Sinclair and Valentine, St. Paul, Minn.) and 20 parts of AFFLAIR 235 rutile green pearl interference pigment from EM Industries, Inc., Hawthorne, N.Y. The composite was then, as in Example 2, converted into a retroreflective sheeting that was adhered to the face of a driver license. The resulting "flip-flop" image was more strongly colored than was that obtained in Example 2, but printed information on the face of the driver license was readable through the image.

EXAMPLE 6

A one-part, solvent-free, radiation addition-polymerizable, crosslinkable, organic oligomeric composition having hard segments and soft segments and a viscosity of 400 cps was prepared by blending together the following components:

    ______________________________________                                                              Parts                                                     ______________________________________                                         acrylate-capped polycaprolactone urethane                                                             48.3                                                    oligomer                                                                       N-vinyl caprolactam    14.0                                                    Isooctyl acrylate      14.0                                                    1,6-hexanediol diacrylate                                                                             22.7                                                    1-hydroxy cyclohexyl acetophenone                                                                     1.0                                                     ______________________________________                                    

Used as an initial master surface was the metallic reflective layer of an enclosed-lens retroreflective sheeting comprising a monolayer of glass microspheres having an average diameter of about 45 micrometers. Onto the metallic layer was deposited the liquid oligomeric composition in an amount barely sufficient to fill the cavities of the metallic layer. This was overlaid with clear PVC film 0.150 mm in thickness through which the oligomeric composition was irradiated using 2 passes of a medium-pressure mercury lamp of 350 to 380 nanometers, 125 watts per linear cm, thus providing a total exposure of 100 millijoule/cm². The contoured surface of the resulting first composite consisted of substantially contiguous hemispheroidal concavities.

The same procedure was repeated using the resultant composite as a forming master, thus producing a second composite having a total thickness of about 0.2 mm and consisting of a PVC substrate and an array of substantially hemi-spheroidal microlenses.

Onto the flat surface of the PVC substrate was flexographic-printed a pattern of nacreous pigment ink from the first station of a two station printing press to form a logo. The ink formulation comprised MEARLITE ULTRABRIGHT UWA nacreous pigment particles (60% BiOCl solids in water from the Mearl Corp. of New York, N.Y.), BAYHYDROL 123 polyurethane binder resin (35% solids in water from Mobay Corp. of Pittsburgh, Pa.), and WERNEKE SSR-2-702 red dye solution from Louis O. Werneke Company of Plymouth, Minn. The first two components were mixed first in equal volumes so that the resultant mixture had 30% BiOCl nacreous pigment solids in water. The red dye solution (0.17 g) was then added to the above mixture (22.7 g) and stirred to give a uniform mixture.

A clear BAYHYDROL 123 polyurethane resin solution was placed in the second station of the printing press and printed in continuous fashion over the nacreous pigment logo and flat surface of the PVC substrate. The BAYHYDROL 123 resin solution functioned as a primer coat for a subsequent adhesive lamination process in which a 0.05 mm thick layer of ELVAX 550 hot melt adhesive was laminated over the primer coat. The resultant transparent protective overlay was laminated by its adhesive layer at 82° C. to the face of a driver license bearing printed information.

When viewed through the overlay, that printed information was clearly readable. Under ambient light conditions, the logo appeared as a silver color at specular reflective viewing angles, and pink at transmission viewing angles. When a retroluminometer was used to view the overlay, only the silver color appeared under the collimated light source, instead of the pink color which appeared under the ambient light conditions. It is believed that this thin and transparent overlay would have been very difficult to duplicate with a color copier because of the construction of the overlay and the use of an intricately printed nacreous pigment logo.

EXAMPLE 7

An overlay was made like the overlay of Example 6 except the BAYHYDROL 123 polyurethane resin solution used in the second printing station was replaced by a second ink formulation comprised of MEARLIN HI-LITE red nacreous pigment particles (100% solids of mica platelets coated with titanium dioxide from the Mearl Corp.), BAYHYDROL 123 polyurethane binder resin, and WERNEKE SSG-8-708 green dye solution. The pigment solution (3 g) was first stirred in the polyurethane binder resin solution (12 g). Then, the green dye solution (0.45 g) was added to the mixture to form a uniform suspension of the ink formulation prior to the printing process. Using the second printing station, this ink formulation was printed over the logo in continuous fashion as described in Example 6 and used as a background color.

The performance results were the same as those achieved with the overlay of Example 6 except that the background was green and the red nacreous pigment was not visible under ambient light conditions at all viewing angles because the green dye of the background ink formulation was present in a high enough concentration to mask the color of the red nacreous pigment particles. However, when the overlay was viewed with a retroluminometer such that the collimated light from the retroluminometer struck the overlay at substantially a right angle, the background was red. The logo had the same appearance when viewed under ambient light conditions and with a retroluminometer as described in Example 6. The use of this second ink formulation as a background color makes it even more difficult to duplicate the overlay using a color copier.

EXAMPLE 8

An overlay was made like the overlay of Example 7 except that the ink formulation used for the logo comprised MEARLITE ULTRABRIGHT UWA nacreous pigment particles, WERNEKE SSW-4010 acrylic binder resin, and WERNEKE SSG-8-708 green dye solution. The resultant overlay had very good heat tampering resistance. The overlay was adhered to a driver license as described in Example 6 and heated to about 250° F. (121° C.) in an attempt to remove the overlay from the dry gelatin photo substrate of the license. When the overlay was heated to this temperature and the overlay peeled back, portions of the logo were separated from the remainder of the overlay and remained on the driver license because of the weak adhesive strength of the acrylic binder resin of the logo relative to the polyurethane binder resin of the background layer. The performance results relative to coloration were substantially the same as those achieved with the overlay of Example 7 except that under ambient light conditions, the logo appeared as a silver color at specular reflective viewing angles, and green at transmission viewing angles. When a retroluminometer was used to view the overlay, only the silver color appeared under the collimated light source.

Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. 

What is claimed is:
 1. A method for making a base sheet, comprising:a) preparing a curable composition; b) depositing said composition onto a forming master surface having a nonuniform array of substantially hemi-spheroidal concavities of different depths therein, said concavities being wider than portions of true hemi-spherical concavities of the same depth; c) spreading a bead of the composition between a transparent substrate film and said forming master, said substrate film being substantially planar, at least one of said forming master and said substrate film being flexible; d) curing the deposited composition to yield a composite comprising a nonuniform array of substantially hemi-spheroidal microlenses of different heights bonded to said substrate film; and e) removing said composite from said forming master to yield said base sheet.
 2. The method of claim 1 wherein said curable composition is an ultraviolet-curable, one-part, solvent-free, radiation addition-polymerizable, crosslinkable, organic oligomeric composition containing hard segments and soft segments and wherein said curing comprises exposure to ultraviolet radiation.
 3. The method of claim 2 wherein said hard segments are polyurethane and said soft segments are polyester.
 4. The method of claim 1 wherein said composition does not extend above the surface of said forming master by a distance greater than 20 percent of the average depth of said hemi-spheroidal concavities.
 5. The method of claim 1 further comprising:f) applying a specularly reflective layer over the exposed face of said substrate film to yield a retroreflective sheeting.
 6. The method of claim 5 further comprising:g) applying a layer of adhesive over said reflective layer.
 7. The method of claim 1 further comprising:a₁) providing an initial master comprising a retroreflective sheeting, said sheeting comprising a monolayer of microspheres and a space coat bearing a metallic coating and covering said monolayer of microspheres, said metallic coating comprising a surface having a nonuniform array of substantially hemi-spheroidal protrusions of different heights thereon, said protrusions being wider at their bases than portions of true hemi-spherical protrusions of the same height; a₂) preparing a second curable composition; a₃) depositing said second composition onto said metallic coating in an mount at least sufficient to cover said protrusions; a₄) spreading a bead of said second composition between a second substrate film and said initial master, said second substrate film being substantially planar, at least one of said initial master and said second substrate film being flexible; a₅) curing said second composition to yield a second composite comprising said second substrate film and said second composition; and a₆) removing said second composite from said initial master to yield said forming master.
 8. A method for making a forming master, comprising:a) providing an initial master comprising a retroreflective sheeting, said sheeting comprising a monolayer of microspheres and a space coat bearing a metallic coating and covering said monolayer of microspheres, said metallic coating comprising a surface having a nonuniform array of substantially hemi-spheroidal protrusions of different heights thereon, said protrusions being wider at their bases than portions of true hemispherical protrusions of the same height; b) preparing a curable composition; c) depositing said composition onto said metallic coating in an amount at least sufficient to cover said protrusions; d) spreading a bead of said composition between a substrate film and an initial master, said substrate film being substantially planar, at least one of said initial master and said substrate film being flexible; e) curing said composition to yield a composite comprising said substrate film and said composition; and f) removing said composite from said initial master to yield said forming master.
 9. The method of claim 8 wherein said protrusions are substantially contiguous and arranged in a substantially random arrangement.
 10. The method of claim 8 wherein said curable composition is an ultraviolet-curable, one-part, solvent-free, radiation addition-polymerizable, crosslinkable, organic oligomeric composition containing hard segments and soft segments and wherein said curing comprises exposure to ultraviolet radiation.
 11. The method of claim 10 wherein said hard segments are polyurethane and said soft segments are polyester. 